Topology induced plasma enhancement for etched uniformity improvement

ABSTRACT

A plasma discharge electrode having a front surface with a central portion thereof including outlets for discharging reactant gas which forms a plasma and a peripheral portion substantially surrounding the outlets. The peripheral portion has at least one recess for locally enhancing a density of the plasma formed by the electrode. The recess can be formed in a replaceable insert and the electrode can be made from a single crystal of silicon.

This application is a continuation of application Ser. No. 08/153,084,filed Nov. 17, 1993, now U.S. Pat. No. 5,472,565.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to etching with high energyradiation or plasma, and more particularly, to a method and apparatusfor controlling the etching rate at the edge of a wafer in an integratedsemiconductor circuit manufacturing process.

2. Description of the Related Art

Since the mid-1960s, integrated semiconductor circuits have become theprimary components of most electronics systems. These miniatureelectronic devices may contain thousands of the transistors and othercircuits that make up the memory and logic subsystems of microcomputercentral processing units. The low cost, high reliability, and speed ofthese computer chips has led them to become a ubiquitous feature ofmodem digital electronics.

The fabrication of an integrated circuit chip typically begins with athin, polished slice of high-purity, single-crystal semiconductormaterial substrate (such as silicon or germanium) called a "wafer." Eachwafer is subjected to a sequence of physical and chemical processingsteps that form the various circuit structures on the wafer. During thefabrication process, various types of thin films may be deposited on thewafer using various techniques such as thermal oxidation to producesilicon dioxide films, chemical vapor deposition to produce silicon,silicon dioxide, and silicon nitride films, and sputtering to produceother metal films.

For example, U.S. Pat. Nos. 4,610,774 ("the '774 patent") and 4,724,060("the '060 patent") to Sakata et al. disclose a target for sputteringand a sputtering apparatus for use in integrated circuit manufacturing.FIG. 2 of the '774 patent illustrates a planar magnetron sputteringelectrode device for forming a composite film on a substrate. Thesputtering electrode has a projecting middle annular target member madeof molybdenum, an outer annular target member made of silicon, and adisc-like inner target member which is also made of silicon. The middletarget may also include an erosion member in the form of a V-shapedgroove, W-shaped, rectangular shaped, or double rectangular shapedgroove, as illustrated in FIGS. 1, 5, 6, and 7 of the '774 patent. FIG.8 of the '774 patent illustrates another planar magnetron sputteringelectrode including a wall provided at a circumferential positionsurrounding the outer silicon target member. It is disclosed that thewall may be maintained at the same, or lower, electric potential as themajor surface of each target member in order to repulse and returnelectrons which tend to escape from the target area. As a result, theplasma density within the outer region can be increased in order toenhance the sputtering yield associated with an annular plasma at theouter position.

FIG. 1 of the '060 patent shows a planar magnetron sputtering apparatusincluding a target member having a plurality of recesses so that thesputtering particles are blocked by the side walls of the recesseswhereby an even film may be deposited on a stepped substrate withoutforming any overhanging shapes on the substrate surface. The recessesmay take on various shapes as illustrated in FIGS. 5-11 of the '060patent.

After depositing a film on the semiconductor wafer, the uniqueelectrical properties of semiconductors are produced by substitutingselected impurities into the semiconductor crystal lattice using aprocess called doping. The doped silicon wafer may then be uniformlycoated with a thin layer of photosensitive, or radiation sensitive,material called a "resist." Small geometric patterns defining theelectron paths in the circuit may then be transferred onto the resistusing a process known as lithography. During the lithographic process,the integrated circuit pattern may be drawn on a glass plate called a"mask" and then optically reduced, projected, and transferred onto thephotosensitive coating covering the resist.

The lithographed resist pattern is then transferred onto the underlyingcrystalline surface of the semiconductor material through a processknown as etching. Traditional etching processes employed wet chemicalsthat proved to be limited in terms of the size and aspect ratio (i.e.,the height to width ratio of the resulting notch) of the features thatcould be formed on the wafer. Consequently, the number of circuits thatcould be packed onto a single wet-etched wafer, and hence the ultimatesize of the electronic device, were limited by traditional chemicaletching processes.

Dry plasma etching, reactive ion etching, and ion milling techniqueswere later developed in order to overcome the limitations associatedwith chemical etching. Plasma etching, in particular, allows thevertical etch rate to be made much greater than the horizontal etch rateso that the resulting aspect ratio of the etched features can beadequately controlled. In fact, plasma etching enables very finefeatures with high aspect ratios to be formed in films approaching 1micrometer in thickness.

During the plasma etching process, a plasma is formed above the maskedsurface of the wafer by adding large amounts of energy to a gas at lowpressure. This is commonly accomplished by creating electricaldischarges in gases at about 0.001 atmospheres. The resulting plasma maycontain ions, free radicals, and neutral species with high kineticenergies. By adjusting the electrical potential of the substrate to beetched, the charged particles in the plasma can be directed to impingeupon the unmasked regions of the wafer and thereby knock out atoms inthe substrate.

The etching process can often be made more effective by using gases thatare chemically reactive with the material being etched. So called"reactive ion etching" combines the energetic etching effects of theplasma with the chemical etching effect of the gas. However, manychemically active agents have been found to cause excessive electrodewear.

It is desirable to evenly distribute the plasma over the surface of thewafer in order to obtain uniform etching rates over the entire surfaceof the wafer. For example, U.S. Pat. Nos. 4,792,378 and 4,820,371 toRose et al. disclose a shower head electrode for distributing gasthrough a number of holes in the electrode. These patents generallydescribe a gas dispersion disk having an arrangement of apertures whichis tailored to the particular pressure gradients existing within areactor chamber in order to provide a uniform flow of gas vapors to asemiconductor wafer. The gas dispersion disk is intended to function asa selective barrier to counteract gradient pressures below the disk andto provide a uniform flow through the shower head electrode fordistribution over the entire surface of the wafer.

Since the integrated circuit fabrication process is quite sensitive toboth particulate and impurity contamination, even airborne particulatematter as small as 1 micrometer must be prevented from contacting thesurface of the wafer during the etching process. Consequently, it isoften desirable to confine the plasma to the area which is immediatelyover and around the wafer substrate. However, physically confining theplasma in this manner has been found to cause reduced plasma densitiesat the edges of the plasma discharge, thus reducing etching rates nearthe edges of the wafer. For example, the '774 patent discloses anannular wall around a sputtering target that can repulse, and returntoward the center, those electrons which tend to escape from the plasmaduring the sputtering process. Any discontinuities or irregularities inthe plasma discharge system, such as pumping ports, may also affect thedensity of the plasma being discharged.

Conventional attempts to uniformly distribute confined plasmas, or toredistribute the plasma near the edges of the electrode, have been foundto cause other undesirable changes in etching process parameters such asetch selectivity and etch profile. Furthermore, even after optimizingprocess variables such as pressure, power, and gas flow, it is oftenimpossible compensate for this edge effect without adversely affectingother process variables. Therefore, there is a significant need in theart to control the plasma density, especially near the edges of a plasmadischarge, without upsetting other process parameters such as etchselectivity and etch profile.

SUMMARY OF THE INVENTION

The present invention generally relates to a method and apparatuswhereby topography, or surface contour effects, can be used to enhance alocal plasma density. For example, the present invention may be appliedto plasma etching processes in order to enhance the density of a plasmaat the edge of a plasma shower head electrode of a parallel plate plasmareaction chamber so as to produce a more uniform etch rate withoutcausing undesirable changes in etching process parameters such as etchselectivity and etch profile. Although the present invention will bedescribed in terms of its advantages for semiconductor manufacturingprocesses, the present invention is also useful for other fields thatmay require plasmas with uniform densities such as plasma ashing, plasmaenhanced chemical vapor deposition, plasma chromatography, plasmadiagnostics, and nuclear fusion.

The present invention provides a plasma discharge electrode which can bemade from a silicon single crystal having a central portion and aperipheral portion which locally enhances a density of a plasma formedby the electrode. The peripheral portion can include at least onerecessed portion in a surface of the electrode facing a semiconductorwafer.

The recessed portion may include one or more grooves extending part wayor completely around the central portion of the electrode. For example,two concentric grooves can extend substantially around the centralportion of the electrode. A width of the groove may be between 5 and 50times a mean free path of a molecule in a gas discharged from theelectrode. The shape of the groove can be U-shaped, V-shaped,channel-shaped, or any combination thereof. The groove may also includeone or more inlets for delivering a gas to the peripheral portion of theelectrode and the groove may be formed in a replaceable insert mountedin the peripheral portion of the electrode.

The recessed portion of the plasma discharge electrode may furtherinclude a plurality of depressions arranged around the peripheralportion of the electrode. A width of the depressions is preferablybetween 5 and 50 times a mean free path of a molecule in the plasmadischarged from the electrode. A shape of at least one of thedepressions may be selected from the group consisting of cylindrical,conical, elliptical, hemispherical, rectangular, or any combinationthereof. At least one of the depressions may include means fordelivering a gas to the peripheral portion of the electrode. At leastone of the depressions may be formed in a replaceable insert which ismounted in the peripheral portion of the electrode.

The electrode and/or the insert may be formed of any suitable materialsuch as graphite, silicon, and aluminum.

The present invention also relates to a method of treating an articlewith a plasma including the steps of forming plasma between an articleand the discharge electrode, the electrode having a peripheral portionincluding at least one recessed portion. The method includes locallyenhancing the density of the plasma adjacent the peripheral portion. Themethod may also include a step of injecting gas into the recessedportion.

The present invention offers several advantages over conventionaltechnology. First, it allows for the localized enhancement, orintensification, of the density of the plasma which is discharged nearthe edge of the silicon wafer in order to compensate for normaldiffusion losses. The shape, dimension, and number of recessed portions,grooves, or depressions may be optimized to provide uniform etching fora variety of different process regimes. Furthermore, the etch uniformitymay be optimized without changing, interfering, or interacting withother process characteristics such as etch selectivity and etch profile.Moreover, the present invention is simpler, less expensive, and morestable than other techniques for locally enhancing plasma density suchas magnetic confinement, RF coupling impedance, and material selection.The present invention is also suitable for many different electrodematerials, plasma, and process gases due to its simple geometry which iseasy to manufacture and use with various insert materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described with reference to thefollowing figures.

FIG. 1 shows a plan view of a conventional shower head electrode;

FIG. 2 shows a cross section taken along section line 2-2' of theconventional shower head electrode in FIG. 1;

FIG. 3 shows a partial cross section from detail "A" in FIG. 2;

FIG. 4 shows a partial cross section of a plasma discharge electrode inaccordance with the invention including a channel-shaped groove;

FIG. 5 shows a partial cross section of a plasma discharge electrode inaccordance with the invention including two channel-shaped grooves;

FIG. 6 shows a partial cross section of a plasma discharge electrode inaccordance with the invention including a V-shaped groove;

FIG. 7 shows a partial cross section of a plasma discharge electrode inaccordance with the invention including a hemispherical groove;

FIG. 8 shows a partial cross section of a plasma discharge electrode inaccordance with the invention including a combination V-shaped andchannel-shaped groove;

FIG. 9 shows a partial cross section of a plasma discharge electrode inaccordance with the invention including a reversed combination V-shapedand channel-shaped groove;

FIG. 10 shows a plan view of a plasma discharge electrode in accordancewith the present invention including a segmented groove;

FIG. 11 shows a plan view of a plasma discharge electrode in accordancewith the present invention including a plurality of depressions;

FIG. 12 shows a cross section along section line 12-12' in FIG. 11; and

FIG. 13 shows a partial cross section from detail "B" in FIG. 12including a replaceable insert.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the present invention, it is possible toadvantageously apply a "hollow cathode" effect toward solving problemsassociated with prior art plasma discharge electrodes of the shower headtype.

The hollow cathode effect is a phenomenon by which the intensity of aplasma discharge can be greatly increased inside an enclosed volume oflimited size, such as a hollow cylinder or tube. It has been found thatthe intensity of the plasma may be most significantly enhanced when thewidth of the enclosed volume is on the order of five to fifty times themean free path of a molecule in the plasma. As is well known in the art,the mean free path (sometimes abbreviated as "mfp") is the distance amolecule travels before collision with another molecule. For example,Argon gas at 300 m Torr and 20° C. has a mean free path of 0.003 inchesand a resulting hollow cathode effect can be obtained with an enclosedvolume of about 0.015 to 0.150 inches.

By opening various portions of an enclosed volume in a hollow cathode,the intensified plasma may be accurately controlled and uniformly aimedat a target. For example, although a round opening at the end of acylindrical enclosure has been found to greatly enhance the density ofplasma which is discharged through the opening, a rectangular openingwhich is perpendicular to the end of the tube has been found to produceless density enhancement with greater uniformity at the edges of thedischarged plasma. Consequently, it has been found that the density andedge uniformity of a plasma being discharged from a hollow cylindricalcathode may be controlled by varying the shape of the opening from thehollow cathode, such as by changing the angle of truncation at the endof the cylinder from round, to elliptical, to rectangular.

FIG. 1 shows a plan view of a conventional shower head electrode 2 whileFIG. 2 shows a cross section taken along section 2-2' of theconventional shower head electrode 2 shown in FIG. 1. A shower headelectrode is one type of plasma electrode that may be used with thepresent invention. The conventional shower head electrode 2 includes aplurality of outlets 4 in a central portion of a front surface 6 fordischarging reactant gas from the rear surface into an area near thefront surface 6 of the electrode 2. The front surface 6 may include anysurface which is not discontinuous with the center of the front surface6 up to coner 7. A peripheral portion of the front surface 6, which isillustrated in FIG. 1 without any outlets 4, surrounds the centralportion of the front surface 6.

Plasma is typically formed by adding large amounts of energy to a lowpressure gas exiting the front surface 6 of the conventional shower headelectrode 2. The plasma is then typically biased toward a semiconductorwafer (not shown) for etching the masked surface of the wafer.

FIG. 3 shows a partial cross section from detail "A" in FIG. 2.

FIG. 3 illustrates the front surface 6 of the conventional electrode 2which is uniformly flat and includes outlets 4 in the central portion offront surface 6. Alternatively, the conventional shower head electrode 2may include a curved front surface 6 which has approximately the sameradius of curvature across the entire front surface 6. In other words,with the conventional shower head electrode 2 illustrated in FIG. 1, thetopography of the front surface 6 is substantially uniform, even nearthe peripheral portion, or outer edge, of the front surface 6.

FIG. 4 shows a partial cross section of a plasma discharge electrode 8in accordance with the one embodiment of the present invention. FIG. 4illustrates the front surface 10 having a nonuniform topography near aperipheral edge portion 12 of the front surface 10. A nonuniformtopography is used here to mean a variation in the surface features onthe front surface 10 of the electrode 8. For example, since the frontsurface on a conventional shower head electrode includes a flat surfacewith a plurality of small outlets, a nonuniform topography would be onethat is not flat and/or includes other surface features besides suchoutlets.

The nonuniform topography at the peripheral edge portion 12 of the frontsurface 10 helps to topographically enhance a local density of a plasmawhich is formed adjacent the front surface 10 of the electrode 8.Although a plasma may be formed simultaneously over both central portion14 and the peripheral portion 12 of the electrode, the topographicallyenhanced density of the plasma may be stronger adjacent peripheral edge12 of the electrode. This local plasma enhancement may be caused, atleast in part, by the hollow cathode effect described above.

Although any appropriate nonuniform topography may be used with thepresent invention, FIGS. 4-13 illustrate various nonuniform topographiesincluding at least one recessed portion in the front surface 10 of theelectrode 8. For example, FIG. 4 illustrates a channel-shaped groove 18which may extend substantially around, or encircle, the peripheral edge12 of the front surface 10 of the electrode 8. The channel-shaped groove18 may have a square bottom, as illustrated in FIG. 4, or another shapesuch as a curved bottom.

FIG. 5 shows a partial cross section including two channel-shapedgrooves 20. In fact, any number of recessed portions, such aschannel-shaped grooves 20 may be arranged to extend around theperipheral portion 12 or perimeter, of the front surface 10 of theelectrode. The recessed portion may be formed from a variety of shapessuch as a V-shaped groove 22 as illustrated in FIG. 6 or a U-shapedgroove 24 as illustrated in FIG. 7. The V-shaped groove helps allow forimproved performance over an extended range of pressures for the gasbeing discharged from the electrode. Alternatively, the recessed portionmay be formed from a combination of shapes such as the combinationV-shaped and channel-shaped groove 26 in FIG. 8 and the reversecombination V-shaped and channel-shaped groove 28 in FIG. 9. The shapeof the various grooves may be chosen to compensate for erosive wear ofthe electrode 8 near the groove.

The width of the grooves may be chosen in accordance with reactortemperature, plasma reactant gas composition and/or reactor pressure soas to be 5 to 50 times the mean free path of a molecule in the plasmaformed over the electrode. For example, molecules in an argon plasma at300 m Torr and 20° C. have a mean free path of about 0.003 inches and insuch a case the groove may have a width of about 0.015 to 0.150 inch.

Although the recessed portions illustrated in FIGS. 4-9 may extendsubstantially around the peripheral edge 12 or perimeter of the frontsurface 10 of the electrode 8, it is also possible to include severalrecessed portions which extend only part way around the perimeter of theelectrode 8. For example, FIG. 10 shows a plan view of a plasmadischarge electrode in accordance with the present invention including asegmented groove 30. Although FIG. 10 shows a groove 30 which has beensegmented into four roughly equal parts, any number of parts in anyarrangement may also be used. The segmented groove 30 may be formed in avariety of shapes, such as those illustrated in FIGS. 4-9.

FIG. 11 shows a plan view of a plasma discharge electrode in accordancewith the present invention including a plurality of depressions 32arranged around the perimeter or peripheral portion 12 of the frontsurface 10. Although the depressions may be evenly distributed aroundthe perimeter of front surface 10 as in FIG. 11, the depressions 32 mayalso be arranged in other patterns. The shape of the depressions 32 maybe shapes such as cylindrical, elliptical, conical, hemispherical,rectangular, or any combination thereof. Consequently, the cross sectionof the depressions 32 may appear similar to those illustrated in FIGS. 4through 9 for the grooves. The shape of the various depressions 32 mayalso be chosen to compensate for any erosive wear of the electrode 8near the depressions 32. As illustrated in FIG. 12, the depressions 32may include one or more inlets 34 for allowing reactant gas to flow froma chamber such as a baffle chamber behind the electrode 8.Alternatively, the inlets 34 may receive reactant gas from anothersource, for example, using a conduit (not shown). A variety of differentgasses and/or reactant agents may thus be introduced into thedepressions 32. Controlling the amount and types of gasses which aresupplied to the front surface 10 through inlets 34 and depressions 32helps allow for better control of the plasma. The peripheral grooves,which were discussed above, may also include similar means fordelivering gas, and/or other fluids, to the front surface 10 via therecessed grooves.

FIG. 13 shows a partial cross section from detail "B" in FIG. 12.

FIG. 13 illustrates a replaceable insert 36 which may be used to formthe depressions 32 with inlets 34 in the electrode 8. The insert 36 mayalso be used with depressions that do not have inlets 34. Likewise, theperipheral grooves, discussed above may include similar inserts forforming the recessed portions in FIGS. 4-10. The inserts 36 may bechosen from a material which is consumed during the plasma dischargeprocess or from a material which is resistant to wear. Using areplaceable insert helps provide for the use of normally incompatiblechemistries such as an oxygen etching gas and a graphite electrode. Theinserts 36 may help protect the electrode 8 from being eroded by plasmaand/or reactant fluid flowing through depressions 32 and any conduitconnected thereto.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. An electrode for use in a plasma reactionchamber, comprising:one or more outlets in a central portion of theelectrode for discharging reactant gas outwardly of an exposed surfaceof the electrode; and a plasma enhancing cavity in the exposed surface,the cavity being located at a peripheral portion of the electrode andextending at least partly around the central portion of the electrode,the cavity being effective for topographically enhancing a local densityof the plasma formed adjacent the exposed surface of the electrode. 2.The plasma electrode as claimed in claim 1, wherein said cavitycomprises at least one recess in said exposed surface of the electrode.3. The plasma electrode as claimed in claim 2, wherein said at least onerecess includes at least one groove extending substantially around saidcentral portion of the electrode.
 4. The plasma electrode as claimed inclaim 2, wherein said at least one recess includes at least twoconcentric grooves extending substantially around said central portionof the electrode.
 5. The plasma electrode as claimed in claim 3, whereina width of said groove is between 5 and 50 times a mean free path of amolecule of the plasma formed by the electrode.
 6. The plasma electrodeas claimed in claim 3, wherein a shape of said at least one groove isselected from the group consisting of U-shaped, V-shaped andchannel-shaped grooves.
 7. The plasma electrode as claimed in claim 3,wherein said at least one groove includes means for discharging gas fromsaid peripheral portion of the electrode.
 8. The plasma electrode asclaimed in claim 3, wherein said at least one groove is formed in areplaceable insert mounted in said peripheral portion of the electrode.9. The plasma electrode as claimed in claim 4, wherein one of said twoconcentric grooves includes means for discharging gas from saidperipheral portion of the electrode.
 10. The plasma electrode as claimedin claim 9, wherein said two concentric grooves are formed in areplaceable insert mounted in said peripheral portion of the electrode.11. The plasma electrode as claimed in claim 2, wherein said at leastone recess includes a plurality of closed surface depressions arrangedaround said peripheral portion of the electrode.
 12. The plasmaelectrode as claimed in claim 11, wherein a width of each of saiddepressions is between 5 and 50 times a mean free path of a molecule ofthe plasma formed by the electrode.
 13. The plasma electrode as claimedin claim 11, wherein a shape of at least one of said depressions isselected from the group consisting of cylindrical, conical, elliptical,hemispherical and rectangular depressions.
 14. The plasma electrode asclaimed in claim 11, wherein at least one of said depressions includesmeans for discharging gas from said peripheral portion of the electrode.15. The plasma electrode as claimed in claim 14 wherein at least one ofsaid depressions is formed in a replaceable insert mounted in saidperipheral portion of the electrode.
 16. The plasma electrode as claimedin claim 11, wherein each of the depressions comprises a fluidimpervious surface extending inwardly into the exposed surface.
 17. Theplasma electrode as claimed in claim 11, wherein each of the depressionshas a width of between 5 and 50 times a mean free path of a molecule ofthe reactant gas.
 18. The plasma electrode as claimed in claim 1,wherein the electrode comprises a single crystal of silicon.
 19. Theplasma electrode as claimed in claim 1 wherein the electrode forms partof a semiconductor etching apparatus.